Test structure for determination of tsv depth

ABSTRACT

A test structure for a through-silicon-via (TSV) in a semiconductor chip includes a first contact, the first contact being electrically connected to a first TSV; and a second contact, wherein the first contact, second contact, and the first TSV form a first channel, and a depth of the first TSV is determined based on a resistance of the first channel.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 12/566,726, filed on Sep. 25, 2009.

BACKGROUND

This disclosure relates generally to the field of semiconductor chip fabrication and testing.

A through-silicon-via, or TSV, provides electrical continuity between the top and bottom surfaces of a semiconductor chip. A TSV is fabricated by deep etching into the silicon wafer, or substrate, that comprises the semiconductor chip, and filling the resulting hole with a liner and a metal filling. The silicon substrate is then ground, or thinned, from the backside until the metal filling is exposed, and backside metal (BSM) is disposed on the thinned backside surface for electrical contact. If the silicon is not etched deeply enough, the TSV may be defective. However, the TSV is not electrically functional before silicon thinning and BSM deposition are completed, therefore, conventional inline electrical testing cannot be used to determine whether the TSV is defective until these steps are completed. Because TSV yield problems may only be detected after the additional process steps are completed, the throughput, efficiency and cost of the semiconductor fabrication process may be negatively impacted.

SUMMARY

An exemplary embodiment of a test structure for a through-silicon-via (TSV) in a semiconductor chip includes a first contact, the first contact being electrically connected to a first TSV; and a second contact, wherein the first contact, second contact, and the first TSV form a first channel, and a depth of the first TSV is determined based on a resistance of the first channel.

Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:

FIG. 1 illustrates a cross section of an embodiment of a semiconductor chip comprising a TSV.

FIG. 2 illustrates a cross section of an embodiment of a test structure channel.

FIG. 3 illustrates a cross section of an embodiment of a test structure channel comprising isolation trenches.

FIG. 4 illustrates an embodiment of a test structure.

FIG. 5 a illustrates a top view of an embodiment of a test structure.

FIG. 5 b illustrates a cross section of an embodiment of a test structure.

FIG. 5 c illustrates a cross section of an embodiment of a test structure.

FIG. 6 a illustrates a top view of an embodiment of a test structure.

FIG. 6 b illustrates a cross section of an embodiment of a test structure.

FIG. 6 c illustrates a cross section of an embodiment of a test structure.

FIG. 7 illustrates an embodiment of a test structure.

FIG. 8 a illustrates a top view of an embodiment of a test structure.

FIG. 8 b illustrates a cross section of an embodiment of a test structure.

FIG. 8 c illustrates a cross section of an embodiment of a test structure.

FIG. 9 a illustrates a top view of an embodiment of a test structure.

FIG. 9 b illustrates a cross section of an embodiment of a test structure.

FIG. 9 c illustrates a cross section of an embodiment of a test structure.

FIG. 10 a illustrates a top view of an embodiment of a test structure.

FIG. 10 b illustrates a cross section of an embodiment of a test structure.

FIG. 10 c illustrates a cross section of an embodiment of a test structure.

FIG. 11 illustrates a method of determining TSV depth.

FIG. 12 illustrates a method of determining TSV depth.

DETAILED DESCRIPTION

Embodiments of structures and methods for determination of TSV depth are provided, with exemplary embodiments being discussed below in detail. A test structure allows determination of TSV depth to be performed before grinding and BSM deposition are performed. In some embodiments, an under-etched TSV may be reworked. The test structure may be implemented in the kerf region of the semiconductor chip.

FIG. 1 illustrates a cross section of an embodiment of a semiconductor chip 100 comprising a TSV. Semiconductor chip 100 comprises silicon substrate 103, oxide layer 105, and back-end-of-line (BEOL) stack 106. The TSV is filled with liner 102 and metal filling 101. The TSV is fabricated by deep etching into silicon substrate 103, in some embodiments to about 150 micrometers (μm) in depth. The depth of the TSV is a function of the etching time. Metal filling 101 may comprise any appropriate conductive metal, including but not limited to tungsten (W), copper (Cu), or aluminum (Al) in some embodiments. BSM 104 is disposed on the backside of silicon 103 after grinding of silicon 103 is performed.

FIG. 2 illustrates an embodiment of a test structure channel 200. TSV 201 is filled with a liner and a metal filling. Heavily doped silicon resistive subcontact 202 and silicide pad 203 comprise a first electrical contact; the first electrical contact is electrically connected to TSV 201. Heavily doped silicon resistive subcontact 204 and silicide pad 205 comprise a second electrical contact. TSV 201 has a depth indicated by line 207, and the distance between TSV 201 and the second electrical contact is indicated by line 208. A current flowing through silicon substrate 206 between the first and second electrical contacts in the channel must flow through TSV 201. The resistance of the channel is proportional to the distance traveled by the current through silicon substrate 206, and may be used to determine the depth 207 of TSV 201. TSV 201 may comprise a test TSV that is etched for use in the test structure in some embodiments; in other embodiments, TSV 201 may comprise a functional TSV that is part of the semiconductor chip circuitry. Test TSVs are formed simultaneously with functional TSVs. Because TSV depth is a function of etching time, test TSVs have the same depth as functional TSVs.

FIG. 3 illustrates an embodiment of a generalized test structure channel 300 comprising isolation trenches 307 and 308. TSV 301 is filled with a liner and a metal filling, and has a depth indicated by line 309. Heavily doped silicon resistive subcontact 302 and silicide pad 303 comprise a first electrical contact; the first electrical contact is electrically connected to TSV 301. Heavily doped silicon resistive subcontact 304 and silicide pad 305 comprise a second electrical contact. A plurality of isolation trenches 307 and 308 are located between TSV 301 and the second electrical contact. Isolation trenches 307 and 308 are formed at the same time as TSV 301 and are not filled with liner and metal. Isolation trenches 307 and 308 may be empty or filled with electrically insulating material such as silicon oxide. Because depth is a function of etching time, isolation trenches 307 and 308 have the same depth 309 as TSV 301. Isolation trenches 307 and 308 are shown for illustrative purposes only, a test structure channel 300 may comprise any appropriate number of isolation trenches. A current flowing through silicon substrate 306 between the first and second electrical contacts in the channel must travel through TSV 301 and underneath isolation trenches 307 and 308. The resistance of the channel is proportional to the distance traveled by the current through silicon substrate 306, and may be used to determine the depth 309 of TSV 301. Isolation trenches 307 and 308 may improve the measurement sensitivity of TSV depth 309. TSV 301 may comprise a test TSV that is etched for use in the test structure in some embodiments; in other embodiments, TSV 301 may comprise a functional TSV that is part of the semiconductor chip circuitry. Test TSVs are formed simultaneously with functional TSVs. Because TSV depth is a function of etching time, test TSVs have the same depth as functional TSVs.

A test structure channel without isolation trenches, such as is shown in FIG. 2, may provide a measurement sensitivity of about 1Ω/μm, while a test structure with isolation trenches, such as is shown in FIG. 3, may have a sensitivity of about 1.5Ω/μm or more. The TSV filling material may comprise copper or tungsten in some embodiments; the choice of filling material has minimal effect on measurement sensitivity. The isolation trenches may be filled with air or silicon oxide in some embodiments. The trench filling material may have an effect on simulated AC reactance, due to the presence of a capacitance effect that is proportional to the dielectric constant of the isolation trench filling material.

FIG. 4 illustrates an embodiment of a test structure 400. Test structure 400 comprises two channels for reference purposes, in order to account for parasitic resistance from contacts or wiring. Points 401 and 402 comprise a first channel, and points 403 and 404 comprise a second channel. The first and second channels are connected to current source 410 via switches 408 and 409, respectively, and to terminals 405, 406 and 407. Terminal 407 is at 0V, and terminal 405 is at voltage ranging between 0.1V and 10V. In the first measurement, switch 408 is closed and switch 409 is opened, such that a current I_(o) from current source 410 flows from point 402 to point 401, and a voltage V₁ is determined at terminal 406. Then switch 409 closed and switch 408 is opened so that I_(o) flows from point 404 to point 403, and a voltage V₂ is then determined at terminal 406. Because current I_(o) is known, the voltage difference (V₁−V₂) indicates the resistance difference between the first channel, which has resistance R₁, and the second channel, which has resistance R₂. I.e., (R₁−R₂)=(V₁−V₂)/I_(o). Since (R₁−R₂) is a function of the TSV depth, the value of (V₁−V₂) may be used to determine TSV depth.

An embodiment of test structure 400 of FIG. 4 in a semiconductor chip is shown in FIGS. 5 a, 5 b, and 5 c. FIG. 5 a illustrates a top view 500 a of test structure 400. TSVs 502 and 505 are filled with a liner and a metal filling. Electrical contact 501 is connected to TSV 502; together 501 and 502 are equivalent to point 401 of FIG. 4. Electrical contact 503 is equivalent to point 402 of FIG. 4. TSV 502 and electrical contact 503 are separated by distance 507. Electrical contact 501, TSV 502, and electrical contact 503 form the first channel. Electrical contact 504 is connected to TSV 505; together electrical contact 504 and TSV 505 are equivalent to point 403 of FIG. 4. Electrical contact 506 is equivalent to point 404 of FIG. 4. TSV 505 and electrical contact 506 are separated by distance 508. Distance 508 may be half of distance 507 in some embodiments. Electrical contact 504, TSV 505, and electrical contact 506 form the second channel. The first channel and the second channel are separated by distance 509. No isolation trench is required between the first and second channels due to the sequential measurements.

FIG. 5 b shows a cross section 500 b of FIG. 5 a along the first channel. Electrical contact 501 is electrically connected to TSV 502, which is separated from electrical contact 503 by distance 507. TSV 502 has a depth 510. FIG. 5 c shows a cross section 500 c of FIG. 5 a along the second channel. Electrical contact 504 is electrically connected to TSV 505, which is separated from electrical contact 506 by distance 508. TSV 505 is of depth 510. A current flowing in the first channel between contact 501 and contact 503 must travel through TSV 502, and a current flowing in the second channel between contact 504 and contact 506 must travel through TSV 505. The resistance of the first and second channels is proportional to the distance traveled by the current through the silicon substrate; the difference in resistance between the two channels may be used to determine the depth 510 of TSVs 502 and 505.

An embodiment of test structure 400 of FIG. 4 in a semiconductor chip is shown in FIGS. 6 a, 6 b, and 6 c. FIG. 6 a illustrates a top view 600 a of test structure 400. TSVs 602 and 605 are filled with a liner and a metal filling. Electrical contact 601 is connected to TSV 602; together electrical contact 601 and TSV 602 are equivalent to point 401 of FIG. 4. Electrical contact 603 is equivalent to point 402 of FIG. 4. TSV 602 and electrical contact 603 are separated by distance 607. Isolation trenches 610-613 are located in between TSV 602 and electrical contact 603. Isolation trenches 610-613 are shown for illustrative purposes only; any appropriate number of isolation trenches may form the first channel. Electrical contact 601, TSV 602, isolation trenches 610-613, and electrical contact 603 form the first channel. Electrical contact 604 is connected to TSV 605; together 604 and 605 are equivalent to point 403 of FIG. 4. Electrical contact 606 is equivalent to point 404 of FIG. 4. TSV 605 and electrical contact 606 are separated by distance 608. Distance 608 may be equal to distance 607 in some embodiments. Electrical contact 604, TSV 605, and electrical contact 606 form the second channel. The first channel and the second channel are separated by distance 609.

FIG. 6 b shows a cross section 600 b of FIG. 6 a along the first channel. Electrical contact 601 is electrically connected to TSV 602, which is separated from electrical contact 603 by distance 607. Isolation trenches 610-613 are located in between TSV 602 and electrical contact 603. TSV 602 and isolation trenches 610-613 have a depth 614. FIG. 6 c shows a cross section 600 c of FIG. 6 a along the second channel. Electrical contact 604 is electrically connected to TSV 605, which is separated from electrical contact 606 by distance 608. TSV 605 is of depth 614. A current flowing in the first channel between contact 601 and contact 603 must travel through TSV 602 and isolation trenches 610-613, and a current flowing in the second channel between contact 604 and contact 606 must travel through TSV 605. The resistance of the first and second channels is proportional to the distance traveled by the current through the silicon substrate; the difference in resistance between the two channels may be used to determine the depth 614 of TSVs 602 and 605.

FIG. 7 illustrates an embodiment of a test structure 700. Test structure 700 comprises two channels for reference purposes, in order to account for parasitic resistance from contacts or wiring. Points 701 and 702 comprise the first channel, and points 703 and 704 comprise the second channel. The first and second channels are connected to identical current sources 709 and 710, respectively, and to terminals 706, 707, and 708. Current sources 709 and 710 each produce current I_(o). Terminal 708 is at 0V, and terminal 706 is at a voltage ranging between 0.1V and 10V. The voltage across the first channel is V₁, and the voltage across the second channel is V₂. Amplifier 705 comprises differential inputs. The non-inverting input 712 of amplifier 705 is connected to V₁, and the inverting input 711 of amplifier 705 is connected to V₂. The resulting output voltage of amplifier 705 at terminal 707 is equivalent to the voltage difference (V₁−V₂) between the first channel and the second channel. Because current I_(o) is known, the voltage difference (V₁−V₂) may be used to determine the resistance difference between the first channel, which has resistance R₁, and the second channel, which has resistance R₂. I.e., (R₁−R₂)=(V₁−V₂)/I_(o). Since (R₁−R₂) is a function of the TSV depth, the value of (V₁−V₂) at terminal 707 may be used to determine TSV depth.

An embodiment of test structure 700 of FIG. 7 in a semiconductor chip is shown in FIGS. 8 a, 8 b, and 8 c. FIG. 8 a illustrates a top view 800 a of test structure 700. TSVs 802 and 805 are filled with a liner and a metal filling. Electrical contact 801 is connected to TSV 802; together 801 and 802 are equivalent to point 701 of FIG. 7. Electrical contact 803 is equivalent to point 702 of FIG. 7. TSV 802 and electrical contact 803 are separated by distance 807. Isolation trenches 810-813 are located in between TSV 802 and electrical contact 803. Isolation trenches 810-813 are shown for illustrative purposes only; any appropriate number of isolation trenches may form the first channel. Electrical contact 801, TSV 802, isolation trenches 810-813, and electrical contact 803 form the first channel. Electrical contact 804 is connected to TSV 805; together electrical contact 804 and TSV 805 are equivalent to point 703 of FIG. 7. Electrical contact 806 is equivalent to point 704 of FIG. 7. TSV 805 and electrical contact 806 are separated by distance 808. Distance 808 may be equal to distance 807 in some embodiments. Electrical contact 804, TSV 805, and electrical contact 806 form the second channel. The first channel and the second channel are separated by distance 809, which may be at least 10 times distance 807 or distance 808 in some embodiments.

FIG. 8 b shows a cross section 800 b of FIG. 8 a along the first channel. Electrical contact 801 is electrically connected to TSV 802, which is separated from electrical contact 803 by distance 807. Isolation trenches 810-813 are located in between TSV 802 and electrical contact 803. TSV 802 and isolation trenches 810-813 have a depth 814. FIG. 8 c shows a cross section 800 c of FIG. 8 a along the second channel. Electrical contact 804 is electrically connected to TSV 805, which is separated from electrical contact 806 by distance 808. TSV 805 is of depth 814. A current flowing in the first channel between contact 801 and contact 803 must travel through TSV 802 and isolation trenches 810-813, and a current flowing in the second channel between contact 804 and contact 806 must travel through TSV 805. The resistance of the first and second channels is proportional to the distance traveled by the current through the silicon substrate; the difference in resistance between the two channels may be used to determine the depth 814 of TSVs 802 and 805.

An embodiment of test structure 700 of FIG. 7 in a semiconductor chip is shown in FIGS. 9 a, 9 b, and 9 c. FIG. 9 a illustrates a top view 900 a of test structure 700. TSVs 902 and 905 are filled with a liner and a metal filling. Electrical contact 901 is connected to TSV 902; together 901 and 902 are equivalent to point 701 of FIG. 7. Electrical contact 903 is equivalent to point 702 of FIG. 7. TSV 902 and electrical contact 903 are separated by distance 907. Electrical contact 901, TSV 902, and electrical contact 903 form the first channel. Electrical contact 904 is connected to TSV 905; together electrical contact 904 and TSV 905 are equivalent to point 703 of FIG. 7. Electrical contact 906 is equivalent to point 704 of FIG. 7. TSV 905 and electrical contact 906 are separated by distance 908. Distance 908 may be half of distance 907 in some embodiments. Electrical contact 904, TSV 905, and electrical contact 906 form the second channel. The first channel and the second channel are separated by isolation trenches 909-912 in some embodiments. Isolation trenches 909-912 prevent interference between the first channel and second channel. Isolation trenches 909-912 are shown for illustrative purposes only; the test structure may comprise any appropriate number of isolation trenches. In embodiments that do not comprise isolation trenches 909-912, the first and second channels may be separated by a distance of at least 10 times distance 907 in order to reduce interference between the first and second channels.

FIG. 9 b shows a cross section 900 b of FIG. 9 a along the first channel. Electrical contact 901 is electrically connected to TSV 902, which is separated from electrical contact 903 by distance 907. TSV 902 has a depth 913. FIG. 9 c shows a cross section 900 c of FIG. 9 a along the second channel. Electrical contact 904 is electrically connected to TSV 905, which is separated from electrical contact 906 by distance 908. TSV 905 is of depth 913. A current flowing in the first channel between contact 901 and contact 903 must travel through TSV 902, and a current flowing in the second channel between contact 904 and contact 906 must travel through TSV 905. The resistance of the first and second channels is proportional to the distance traveled by the current through the silicon substrate; the difference in resistance between the two channels may be used to determine the depth 913 of TSVs 902 and 905.

An embodiment of test structure 700 of FIG. 7 in a semiconductor chip is shown in FIGS. 10 a, 10 b, and 10 c. FIG. 10 a illustrates a top view 1000 a of test structure 700. TSVs 1002 and 1005 are filled with a liner and a metal filling. Electrical contact 1001 is connected to TSV 1002; together electrical contact 1001 and TSV 1002 are equivalent to point 701 of FIG. 7. Electrical contact 1003 is equivalent to point 702 of FIG. 7. TSV 1002 and electrical contact 1003 are separated by distance 1007. Isolation trenches 1010-1013 are located in between TSV 1002 and electrical contact 1003. Isolation trenches 1010-1013 are shown for illustrative purposes only; any appropriate number of isolation trenches may form the first channel. Electrical contact 1001, TSV 1002, isolation trenches 1010-1013, and electrical contact 1003 form the first channel. Electrical contact 1004 is connected to TSV 1005; together electric contact 1004 and TSV 1005 are equivalent to point 703 of FIG. 7. Electrical contact 1006 is equivalent to point 704 of FIG. 7. TSV 1005 and electrical contact 1006 are separated by distance 1008. Distance 1008 may be equal to distance 1007 in some embodiments. Electrical contact 1004, TSV 1005, and electrical contact 1006 form the second channel. The first channel and the second channel are separated by isolation trenches 1014-1015. Isolation trenches 1014-1015 prevent interference between the first channel and second channel. Isolation trenches 1014-1015 are shown for illustrative purposes only; the test structure may comprise any appropriate number of isolation trenches.

FIG. 10 b shows a cross section 1000 b of FIG. 10 a along the first channel. Electrical contact 1001 is electrically connected to TSV 1002, which is separated from electrical contact 1003 by distance 1007. Isolation trenches 1010-1013 are located in between TSV 1002 and electrical contact 1003. TSV 1002 and isolation trenches 1010-1013 have a depth 1009. FIG. 10 c shows a cross section 1000 c of FIG. 10 a along the second channel. Electrical contact 1004 is electrically connected to TSV 1005, which is separated from electrical contact 1006 by distance 1008. TSV 1005 is of depth 1009. A current flowing in the first channel between contact 1001 and contact 1003 must travel through TSV 102 and underneath isolation trenches 1010-1013, and a current flowing in the second channel between contact 1004 and contact 1006 must travel through TSV 1005. The resistance of the first and second channels is proportional to the distance traveled by the current through the silicon substrate; the difference in resistance between the two channels may be used to determine the depth 1009 of TSVs 1002 and 1005.

FIG. 11 illustrates an embodiment of a method of testing TSV depth in a semiconductor chip. In block 1101, one or more TSVs are etched in the semiconductor chip. In some embodiments, isolation trenches also etched into the semiconductor chip simultaneously with the TSVs. In block 1102, the TSVs are filled with a liner and metal filling. In block 1103, a test structure is formed around the TSVs. The test structure comprises electrical contacts; each electrical contact comprises a heavily doped silicon resistive subcontact and a silicide pad. The test structure may correspond to one of the embodiments discussed above with regards to FIGS. 4-10 c. In block 1104, the TSV depth is determined using the test structure. In block 1105, if the TSV depth is determined to be correct (i.e., meets a predetermined threshold), grinding and application of BSM are performed on the backside of the semiconductor chip.

FIG. 12 illustrates an embodiment of a method of testing TSV depth in a semiconductor chip. In block 1201, TSVs are etched in the semiconductor chip. Test TSVs and functional TSVs are etched simultaneously; because TSV depth is a function of etching time, the test TSVs and the functional TSVs have the same depth. In some embodiments, isolation trenches also etched into the semiconductor chip simultaneously with the test and functional TSVs. The test TSVs and the isolation trenches may be formed in the kerf region of the semiconductor chip. In block 1202, the functional TSVs are masked, and the test TSVs are filled with a liner and a metal filling. In embodiments comprising isolation trenches, the isolation trenches are also masked. In block 1203, a test structure is formed around the test TSVs. The test structure comprises electrical contacts; each electrical contact comprises a heavily doped silicon resistive subcontact and a silicide pad. The test structure may correspond to one of the embodiments discussed above with regards to FIGS. 4-10 c. In block 1204, the depth of the test TSVs is determined using the test structure. In block 1205, in the event the test TSVs are determined not to be deep enough (i.e., do not meet a predetermined threshold), the functional TSVs are further etched to the correct depth. In block 1206, filling of the functional TSVs with a liner and metal filling, and grinding and application of BSM on the backside of the semiconductor chip, are performed.

The technical effects and benefits of exemplary embodiments include improved yield of semiconductor chips, due to detection of defective TSVs relatively early in the fabrication process.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

1. A test structure for a through-silicon-via (TSV) in a semiconductor chip, comprising: a first contact, the first contact being electrically connected to a first TSV; and a second contact, wherein the first contact, second contact, and the first TSV form a first channel, and a depth of the first TSV is determined based on a resistance of the first channel.
 2. The test structure for a TSV of claim 1, wherein the first contact and the second contact each comprise a heavily doped silicon resistive subcontact and a silicide pad.
 3. The test structure for a TSV of claim 1, further comprising a plurality of isolation trenches located between the first TSV and the second contact.
 4. The test structure for a first TSV of claim 3, wherein the plurality of isolation trenches are filled with one of air and silicon dioxide.
 5. The test structure for a first TSV of claim 3, wherein the first TSV is filled with a liner and a metal filling, the metal filling comprising one of copper, tungsten, and aluminum.
 6. The test structure for a TSV of claim 3, wherein the plurality of isolation trenches have a depth equal to the depth as the first TSV.
 7. The test structure for a TSV of claim 1, further comprising a second channel comprising a second TSV electrically connected to a third contact, and a fourth contact, the second TSV having a depth that is the same as the depth of the first TSV.
 8. The test structure for a TSV of claim 7, further comprising a plurality of isolation trenches located between the first channel and the second channel.
 9. The test structure for a TSV of claim 7, wherein a distance between the first TSV and the second contact is about twice a distance between the second TSV and the fourth contact.
 10. The test structure for a TSV of claim 7, wherein a distance between the first TSV and the second contact is the same as a distance between the second TSV and the fourth contact.
 11. The test structure for a TSV of claim 7, wherein a distance between the first channel and the second channel is about 10 times a distance between the first TSV and the second contact.
 12. The test structure for a TSV of claim 7, further comprising: a current source; a first switch connected between the current source and the first channel; and a second switch connected between the current source and the second channel, wherein in the event the first switch is closed and the second switch is open, a resistance of the first channel is determined, and in the event the first switch is open and the second switch is closed, a resistance of the second channel is determined, and wherein the depth of the first TSV is determined based on the resistance of the first channel and the resistance of the second channel.
 13. The test structure for a TSV of claim 7, further comprising: a first current source connected to the first channel; a second current source connected to the second channel, the second current source providing a current that is the same as a current provided by the first current source; and a differential amplifier connected between the first channel and the second channel, the differential amplifier configured to determine a voltage difference between the first channel and the second channel, wherein a resistance difference between the first channel and the second channel is determined based on the voltage difference between the first channel and the second channel, and wherein the depth of the first TSV is determined based on the resistance difference between the first channel and the second channel.
 14. The test structure for a TSV of claim 1, wherein the test structure is located in a kerf region of the semiconductor chip. 